One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between downhole locations themselves. For example, in the oil and gas industry it is desirable to communicate data generated downhole to the surface during operations such as drilling, perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to downhole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable downhole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded digital signals.
Downhole testing is traditionally performed in a “blind fashion”: downhole tools and sensors are deployed in a well at the end of a tubing string for several days or weeks after which they are retrieved at surface. During the downhole testing operations, the sensors may record measurements that will be used for interpretation once retrieved at surface. It is only after the downhole testing tubing string is retrieved that the operators will know whether the data are sufficient and not corrupted. Similarly when operating some of the downhole testing tools from surface, such as tester valves, circulating valves, packer, samplers or perforating charges, the operators do not obtain a direct feedback from the downhole tools.
In this type of downhole testing operations, the operator can greatly benefit from having a two-way communication between surface and downhole. However, it can be difficult to provide such communication using a cable since inside the tubing string it limits the flow diameter and requires complex structures to pass the cable from the inside to the outside of the tubing. A cable inside the tubing is also an additional complexity in case of emergency disconnect for an offshore platform. Space outside the tubing is limited and a cable can easily be damaged. Therefore a wireless telemetry system is preferred.
A number of proposals have been made for wireless telemetry systems based on acoustic and/or electromagnetic communications. Examples of various aspects of such systems can be found in: U.S. Pat. No. 5,050,132; U.S. Pat. No. 5,056,067; U.S. Pat. No. 5,124,953; U.S. Pat. No. 5,128,901; U.S. Pat. No. 5,128,902; U.S. Pat. No. 5,148,408; U.S. Pat. No. 5,222,049; U.S. Pat. No. 5,274,606; U.S. Pat. No. 5,293,937; U.S. Pat. No. 5,477,505; U.S. Pat. No. 5,568,448; U.S. Pat. No. 5,675,325; U.S. Pat. No. 5,703,836; U.S. Pat. No. 5,815,035; U.S. Pat. No. 5,923,937; U.S. Pat. No. 5,941,307; U.S. Pat. No. 5,995,449; U.S. Pat. No. 6,137,747; U.S. Pat. No. 6,147,932; U.S. Pat. No. 6,188,647; U.S. Pat. No. 6,192,988; U.S. Pat. No. 6,272,916; U.S. Pat. No. 6,320,820; U.S. Pat. No. 6,321,838; U.S. Pat. No. 6,912,177; EP0550521; EP0636763; EP0773345; EP1076245; EP1193368; EP1320659; EP1882811; WO96/024751; WO92/06275; WO05/05724; WO02/27139; WO01/39412; WO00/77345; WO07/095111.
Because of the repetitive structure of piping structure used, the characteristic of the acoustic propagation along pipes is such that the frequency response of the channel is complex. FIG. 13 shows the experimental and theoretical frequency response of a piping structure comprising two pipes below the wave source and eight pipes above. The spectrum has numerous peaks and troughs which are difficult to predict beforehand. Given the spectrum and the use of a mono-carrier modulation scheme, choosing a peak for the carrier frequency of the transmitted modulated signal where noise is incoherent with the signal is advantageous in term of signal to noise ratio. Choosing a carrier frequency around a locally flat channel response, i.e. no distortion, is advantageous to maximize the bit rate. In any case, choosing the carrier frequency in situ is a requirement, and the process of choosing the right frequency may take time and computing resources and has to be as simple as possible.
US 2006/0187755 by Robert Tingley discloses a method and system for communicating data through a drill string by transmitting multiple sets of data simultaneously at different frequencies. The Tingley reference attempts to optimize the opportunity of successful receipt despite the acoustic behavior of the drill string, and thereby avoiding the problem of selecting a single frequency.
Moreover, U.S. Pat. No. 5,995,449 by Clark Robison et al. discloses a method and apparatus for communicating in a wellbore utilizing acoustic signals. However, the Robison et al. disclosure relates specifically to an apparatus and method for transmitting acoustic waves through the completion liquid as a transmission medium, rather than the tubing or pipe string.
It is an object of the present invention to provide a system that allows automatic synchronization of transmitters and receivers on an appropriate frequency for reliable data transmission along tubing in a borehole.